Multimode optical amplifier with close-loop modal gain control

Abstract

Systems and methods for amplification are shown that include a pump preparation module configured to provide a pump output that includes a plurality of pump modes; an amplification module configured to accept a multimode signal input and the pump output, such that the pump output causes an amplification of a plurality of modes in the signal input to produce an amplified signal output; and a gain control module configured to adjust a balance of the plurality of pump modes in the pump output to produce a predetermined amplified signal output.

What is claimed is: 
     
       1. A system for amplification, comprising:
 a pump preparation module configured to provide a pump output that includes a plurality of pump modes; 
 an amplification module configured to accept a multimode signal input and the pump output, such that the pump output causes an amplification of a plurality of modes in the signal input to produce an amplified signal output; and 
 a gain control module configured to adjust a balance of the plurality of pump modes in the pump output to produce a predetermined amplified signal output. 
 
     
     
       2. The system of  claim 1 , wherein the gain control module configured to adjust the balance of the plurality of pump modes according to a functional relationship between pump modes and signal gain to provide configuration values for the pump preparation module that produce the predetermined amplified signal output. 
     
     
       3. The system of  claim 1 , wherein the gain control module comprises a table of pump mode values configured to provide configuration values for the pump preparation module that produce the predetermined amplified signal output. 
     
     
       4. The system of  claim 1 , wherein the amplification modules comprises a region of doped fiber having a differential amplification response that depends on input signal and pump modes. 
     
     
       5. The system of  claim 1 , further comprising a power detector unit comprising a plurality of photodetectors configured to measure the power output of each pump output mode and each amplified signal output mode, configured to provide output power information to the gain control module for use in adjusting the pump mode balance. 
     
     
       6. The system of  claim 1 , wherein the pump preparation module comprises:
 a plurality of mode converters, one for each pump mode output, configured to accept an input and to produce an output that includes only a single pump mode; 
 a plurality of variable optical attenuator, each connected between a pump source and a mode converter, configured to produce a specified power balance between pump modes; and 
 a mode combiner configured to combine the single-mode outputs of the plurality of mode converters into a single multimode pump output. 
 
     
     
       7. The system of  claim 6 , wherein the mode converters are freespace phase masks configured to permit only a single pump mode. 
     
     
       8. The system of  claim 6 , wherein the mode converters are gratings based in multimode fiber configured to permit only a single pump mode. 
     
     
       9. The system of  claim 1 , further comprising a wavelength division multiplexing (WDM) combiner configured to combine the multimode signal input and the pump output into a single output that is provided to the amplification module. 
     
     
       10. The system of  claim 9 , wherein the WDM combiner comprises one of a dielectric stack filter and a half-mirror. 
     
     
       11. The system of  claim 9 , wherein the WDM combiner comprises a multimode fiber combiner. 
     
     
       12. The system of  claim 1 , further comprising a wavelength division multiplexing (WDM) splitter that receives the output of the amplification module and is configured to split the amplified signal output and the pump output into separate outputs. 
     
     
       13. The system of  claim 12 , wherein the WDM splitter is a dielectric stack. 
     
     
       14. The system of  claim 12 , wherein the WDM splitter comprises a half-mirror and a set of optical bandpass filters configured to limit the outputs based on wavelength. 
     
     
       15. The system of  claim 12 , wherein the WDM splitter is a multimode fiber coupler. 
     
     
       16. A system for amplification, comprising:
 a pump preparation module configured to provide a pump output that includes a plurality of pump modes in accordance with configuration values; 
 a wavelength division multiplexing (WDM) combiner configured to combine a multimode signal input and the pump output into a combined output; 
 an amplification module configured to accept the combined output of the WDM combiner, such that the pump output causes an amplification of a plurality of modes in the signal input to produce an amplified signal output; 
 a WDM splitter that receives the output of the amplification module and is configured to split the amplified signal output and the pump output into separate outputs; and 
 a gain control module configured to adjust the balance of the plurality of pump modes in the pump output according to a functional relationship between pump modes and signal gain, providing configuration values for the pump preparation module that produce the predetermined amplified signal output. 
 
     
     
       17. A method for amplification, comprising:
 measuring a power for each mode in an unamplified multimode optical signal input; 
 providing a multimode pump output to an amplifier, wherein the multimode pump output has a balance of mode powers that produce a predetermined differential gain for the optical input signal; and 
 amplifying the optical input signal using an amplification medium and the multimode pump output to produce an amplified multimode optical signal output. 
 
     
     
       18. The method of  claim 17 , further comprising:
 measuring a power of the modes of the amplified multimode optical signal output; 
 adjusting the balance of the multimode pump output if the power of the modes differs from a predetermined target output power. 
 
     
     
       19. The method of  claim 17 , wherein providing the multimode pump output comprises:
 producing a plurality of multimode pump paths; 
 limiting each path to a different pump mode; and 
 adjusting the power of each path to produce the balance of mode powers.